1. Field of the Invention
This invention relates to a method for making low-porosity cement paste. More particularly, this invention relates to a method for producing low porosity cement from a gypsum-free hydraulic cement which includes alkali bicarbonates and lignosulfonates and follows a particular mixing sequence to give a cement paste having an extended set time and other benefits.
Cements are produced by calcining suitable raw materials, generally a mixture of calcareous and argillaceous materials, to produce a sintered "clinker". Portland types are by far the most important cements in terms of quantity produced. The clinker is conventionally mixed with small amounts of gypsum, i.e., up to about 9%, and ground, usually in some type of ball mill, to a finely divided state having a relatively large surface area to yield the finished cement.
The ground clinker containing gypsum is mixed with the proper amount of water to form a paste. Properly made cement pastes set within a few hours and then harden slowly. Cement pastes are combined with aggregates, either fine aggregates or sand to produce mortars or larger aggregates as gravel, stone and the like to produce concrete. The paste acts as the cementing material and its composition has a decisive effect on the strength and other properties of the resultant mortar or concrete.
One of the main factors that determine the properties of hardened cement pastes and, consequently of mortar and concrete, is the water-to-cement ratio of the fresh mix. The lower the water-to-cement ratio the higher the strength, lower the shrinkage, and better the frost and corrosion resistance. The desirability of having a low water-to-cement ratio, the conventional practice being normally between about 0.4 and 0.6, is to obtain a concrete or mortar having minimum shrinkage and increased ultimate strength. However, simply lowering the water-to-cement ratio of conventional portland cements is not the answer. The fact that a decrease of the water content improves the properties of the hardened concrete can be used only to a limited degree as a decrease of the water content results simultaneously in a deterioration of the workability and shortening of the setting time of the concrete mix. The requirements for a sufficient workability of the fresh concrete mix are the reasons for the fact that the water content of concrete mixes used in practical applications lies far above the amount that is needed for complete hydration of cement. While the amount of water needed for complete hydration of cement is about 22-23%, the lowest amount of water used in conventional concrete practice lies in the vicinity of 40% and usually ranges between 45% and 80%.
Even through the use of conventional water reducers (mainly lignosulfonates from spent sulfite pulping liquors) a water reduction of only about 10% of the water added is possible. The water remaining in the concrete mix made from ordinary cement is still far above the requirements needed for complete hydration of the cement. Thus, if the water content could be further reduced without deterioration of the workability or without introducing other detriments, a significant gain in strength and an improvement of some other properties of the hardened concrete could be reached.
2. The Prior Art
Efforts to produce low porosity cements by reduction of water-to-cement ratio have been long attempted. For example, U.S. Pat. No. 2,174,051 to Winkler teaches that an increase in strength is obtained with a low water-to-cement ratio and that certain organic compounds such as tartaric acid, citric acid and the like may be added to regulate the setting time.
U.S. Pat. No. 2,374,581 to Brown teaches that small amounts of tartaric acid, tartartes and bicarbonates may be added to ordinary (gypsum containing) portland cement at conventional water-to-cement ratios to retard the rate of set at high temperatures in the cementing of oil wells.
U.S. Pat. No. 2,646,360 to Lea teaches that an alkali metal or alkaline earth metal lignin sulfonate and an alkali metal salt of an inorganic acid (e.g., sodium carbonate) may be added to a gypsum containing cement slurry to reduce water loss and thus the amount of water initially needed.
U.S. Pat. No. 3,118,779 to Leonard, on the other hand, teaches that sodium bicarbonate when added to a portland cement-Type III (containing gypsum) without lignin being present acts as an accelerator.
U.S. Pat. No. 3,689,296 to Landry teaches that formaldehyde modified calcium lignosulfonates may be used in portland cements to replace all or part of the gypsum usually added and the amount of water required for a mix of a given degree of fluidity is reduced.
U.S. Pat. No. 3,689,294 to Braunauer reflects more recent effort to produce low porosity cements by grinding portland-type cements without gypsum to a specific surface area between 6,000 - 9,000 Blaine (cm..sup.2 /gm.) and mixing with alkali or alkaline earth lignosulfonate, alkali carbonate and water.
U.S. Pat. No. 3,782,984 to Allemand et al. teaches that the addition of 0.5% to 5% of alkali metal acid carbonates to portland-type cements accelerates the setting time.
The French publication Les Adjuvants Du Ciment edited by Albert Joisel (Soisy, France 1973 published by the author) at page 102 teaches that sodium bicarbonate in ordinary portland cement is a retarder and again at page 132 that sodium bicarbonate may be added to portland cement with gypsum in the usual way.
The above described prior art is intended as exemplary and not inclusive of all low porosity cement art.
It is, therefore, a general object of this invention to provide a process for making an improved low porosity, free-flowing cement paste.
Another object of this invention is to provide an improved low porosity, free-flowing cement with reduced expansion due to alkali aggregate reactions by including sodium bicarbonate as an additive.
A further object of this invention is to provide a process for mixing cement, water and adjuvants to obtain a low porosity, free-flowing cement paste having an extended set time and improved workability.
Other objects, features and advantages of this invention will become evident from the following detailed description of the invention.